<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      body {
        margin: 0;
        padding: 0;
        background-image: url(../images/grid.png);
      }
      canvas {
        background-color: rgba(255, 0, 0, 0.1);
      }
    </style>
  </head>
  <body>
    <canvas id="tutorial" width="300" height="300px">
      你的浏览器不兼容Canvas,请升级您的浏览器!
    </canvas>

    <script>
      window.onload = function () {
        let canvasEl = document.getElementById('tutorial')
        if (!canvasEl.getContext) {
          return
        }
        let ctx = canvasEl.getContext('2d') // 2d | webgl

        // 针对于Canvas中所有的图形生效
        ctx.globalAlpha = 0.3

        // 2.修改画笔的颜色
        ctx.fillStyle = 'rgba(255, 0, 0, .3)'
        ctx.fillRect(0, 0, 100, 50) // 单位也是不用写 px

        ctx.fillStyle = 'blue'
        ctx.fillRect(200, 0, 100, 50)

        ctx.fillStyle = 'green' // 关键字, 十六进制, rbg , rgba
        ctx.beginPath()
        ctx.rect(0, 100, 100, 50)
        ctx.fill()
      }
    </script>
  </body>
</html>
